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Monday, June 3, 2013

Water Tank Leaks and Welding - Again

When we picked up LUX in Marsh Harbor for the spring trip back to Annapolis, the local charter maintenance guy told us that the stbd water tank was leaking. He had talked with the local Moorings folks, who told him that they repaired the tanks with fiberglass. To our knowledge, that wouldn't work because the fiberglass won't bond to the High-Density PolyEthylene (HDPE) of the tank.

Sure enough, when we filled the tank, the water slowly leaked out. When we removed the tank, the fiberglass patch popped right off, as shown in the first picture. We noted that our old weld job had held - we are dealing with a new fracture.

As usual, the first step of a repair is doing the preparation. After removing the old fiberglass repair, we used a rotary tool to open the crack to a U or V shape and to remove the old, damaged material, which had a brown color. We used a heavy duty cutoff disc held at an angle to the fracture. The finished prep is in the next picture. You can see the marks left from the rotary tool cutoff disc.

We have a set of HDPE plastic strips from an old milk jug. The jug must have the recycle designation HDPE molded into the bottom. HDPE is also used in potable antifreeze jugs and a variety of other gallon jugs. It is good to only use material from a food-grade source. The white material we have is a nice contrast to the green tank material - we can easily see what work we've done.

Now we are ready to start welding. We use a heavy duty heat gun ($20 from Home Depot) and a Weller hot knife (Sailrite). The heat gun is used to heat the tank and the strip of new material so that they start to turn translucent. Move the heat gun further away at this time, applying just enough heat to keep the material warm but not enough to cause it to sag. Then use the hot knife to press and weld the new strip to the tank material. The localized heating from the hot knife will cause the new strip of HDPE to melt as well as the layer of tank material just below it. There are several videos on the Internet that show plastic welding and watching a few of them is helpful before starting on something important.

As with any welding (plastic or metal), the first layer can be challenging, because there isn't much of the old material left and it may be necessary to bridge a gap. Only heat the materials enough to often them and use the hot knife to weld them together. We started with a narrow strip at the bottom of the repair and worked up to larger width strips. Here is a sequence of photos that show the process of welding a strip of material to the tank.

Cut off the desired strip length with the hot knife and tack down an edge.

Use the heat gun to heat the strip and the tank until the strip is translucent. Press the new strip into the pliable material under neath. Move the hot knife slowly to allow the heat to penetrate the thin strip, welding it to the material underneath.

Use the hot knife to press the new material down onto the softened tank material. Make sure you don't get bubbles trapped under the new strip. If that happens, cut through the strip with the hot knife to open the void, then slowly work material back into the cut using the hot knife. I like to work from the center of the new material out to the edge to help prevent voids.

We're nearly finished at this point, having added multiple layers of HDPE to build up to the original tank thickness. It sometimes helps to hold a board or other backing plate behind the weld to keep it from sagging. If the tank is sagging around the weld, you're using too much heat gun. Let it cool until the HDPE cools enough to become opaque, then start over. Working with small strips allows easy heating of the strip without over heating the tank itself. This is a prime example of many small steps being best.

Practice helps a lot. Food-grade 5-gallon buckets used in restaurant supply are a good source of HDPE as well as a container on which to practice welding.

We thought about adding a big patch over the entire area, but decided that working out the voids that would undoubtably occur was not worth it. That doesn't mean that it isn't a reasonable approach - just that we thought that multiple narrower strips were easier to add and would result in the same strength.

Finally, we taped the hot knife to a length of PVC pipe and smoothed out the weld on the inside of the tank.

Here's a picture of the procedure in progress.

And the final result. Practice has definitely helped, because this is is the best looking weld so far. We used a strip of HDPE that was about 12 inches long and 3/8 to 1/2 inch wide to do this weld.

After a day-long offshore trip from Wrightsville Beach to Beaufort, NC, the weld is holding. So the short-term prognosis is good.